Cellulose fibers and their derivatives constitute the most abundant renewable polymer resource available. Cellulose fibers are also economically viable substances due to their low cost, availability, renewability, and physical properties. As a result, they have been subject to a variety of research for their ability to be applied as reinforcing agents.
Conventionally, cellulose is used as a construction material (wood), as a natural textile (cotton and flax), and for paper and board. Moreover, high performance cellulose-based materials are used throughout industry and everyday life. In all of these applications, cellulose-cellulose and cellulose-polymer interactions are vital, but not well-understood.
Recently, research regarding nanocrystalline cellulose (NCC) has become increasingly popular, particularly because of their renewability and sustainability, as well as their applications as reinforcing agents. NCC can be derived from cellulose fibers by hydrolyzing the amorphous and paracrystalline regions of cellulose fibers by either enzymatic or acidic hydrolysis, and dispersing them in water. The resulting crystalline nanoparticles are exceptionally tough, with an axial Young's modulus that is theoretically similar to that of KEVLAR®, making them desirable for use as reinforcing fillers in composite systems. NCCs also have an abundance of hydroxyl groups and anionically charged functional groups (carboxylate, sulfate) at the surface. This charged hydrogen-bonding surface results in insolubility and poor dispersion in low dielectric media and is expected to result in agglomeration of the NCC when it is incorporated into a fluoropolymer composite. However, the abundance of hydroxyl groups at the NCC surface allows for various chemical modifications to be performed.
One type of chemical surface modification may include functionalization. Many chemical functionalizations have been primarily conducted to (1) introduce stable negative or positive electrostatic charges on the surface of NCCs to obtain better dispersion, and (2) enhance surface energy characteristics to improve compatibility when used in conjunction with non-polar or hydrophobic matrices in nanocomposites.
NCCs present an alternative to fillers (i.e., inorganic, carbon nanotubes) in composite systems owing to its natural abundance, unique material properties, and sustainability. For example, although fluoropolymers are often characterized by their superior chemical and thermal stability and low coefficient of friction, fillers are often required to improve fluoropolymer mechanical strength. However, such fillers are limited in nature. Although NCCs can be an effective alternative to fillers, it remains a challenge to chemically functionalize NCC surfaces in a way such that NCC morphology and crystalline structure are preserved. Further, although NCCs may form stable suspensions in aqueous media, they still cannot be easily dispersed in non-polar solvents or polymers. Thus, there is a need to provide functionalized NCC surfaces to be successfully employed as a reinforcing material in fluoropolymer composites.